Typically, in computed tomography (CT) systems, an x-ray tube emits a fan-shaped x-ray beam or a cone-shaped x-ray beam toward a patient positioned on a table. The x-ray beam, after being attenuated by the patient, impinges upon a detector assembly comprising a plurality of detector elements. The intensity of the attenuated x-ray beam received at the detector assembly is typically dependent upon the attenuation of the x-ray beam by the patient. Each detector element produces a separate electrical signal indicative of the intensity of the attenuated x-ray beam received at that particular detector element.
In known third generation CT systems, the x-ray source and the detector assembly are rotated on a gantry around the object to be imaged so that a gantry angle at which the fan-shaped or cone-shaped x-ray beam intersects the patient constantly changes. The table supporting the patient may be advanced while the gantry is rotating around the object being imaged. Data representing the intensity of the received x-ray beam at each of the detector elements is collected across a range of gantry angles. The data are ultimately reconstructed to form an image of the patient.
Third generation CT systems typically acquire data using either an axial acquisition mode or a helical acquisition mode. During an axial acquisition mode, the patient is stationary with respect to the rotating gantry while an axial dataset is acquired. During a helical acquisition mode, the patient is advanced with respect to the rotating gantry while a helical dataset is acquired.
Since the patient is not moving while the axial dataset is collected, it is possible to use a much simpler reconstruction algorithm to reconstruct an image from the axial dataset. Compared to an image reconstructed from the helical dataset, it may be possible to reconstruct an image with better resolution and fewer artifacts by using an axial dataset. However, when acquiring an axial dataset of an anatomical feature experiencing periodic motion, such as the heart, it is a standard practice to only acquire data during a specific portion of the periodic cycle. This is called prospective gating. Acquiring a prospectively gated axial dataset minimizes the patient's exposure to x-rays since the x-ray beam is only activated during a portion of the patient's periodic cycle.
The helical acquisition mode has a different set of strengths and weaknesses when compared to the axial acquisition mode. Since the table is moving during the helical acquisition, it is possible to cover more of the patient in a z-direction with a helical acquisition mode than with a single axial acquisition. Also, the helical acquisition mode accommodates a wider range of patients more easily. For example, it is difficult to accommodate patients with a highly variable heart rate using a conventional axial acquisition mode. It is also hard to acquire an axial dataset of the correct portion of the cardiac cycle if the patient experiences an irregular heartbeat. Since the helical acquisition mode is not prospectively gated, it easily accommodates both patients with variable heart rates and patients experiencing irregular heartbeats. However, since the helical acquisition mode is not prospectively gated, it may result in exposing the patient to a higher x-ray dose than a prospectively gated axial scan of the same anatomic region.
For these and other reasons, there is a need for an acquisition mode that addresses some of the limitations of the known axial acquisition mode and the known helical acquisition mode.